A conductive plastic is a polymer which has attracted attention again since professors A. J. Heeger and A. G. MacDiarmid in the USA and professor H. Shirakawa in Japan were awarded the Novel Prize in chemistry in 2000. Since they firstly reported in 1977 that polyacetylene as a polymer can carry electricity through a doping process, lots of research on the conductive plastic has been carried out, which can be seen from the fact that the number of disclosed literatures has increased by more than double from 18,000 in 2000 to 42,000 in 2009.
Such conductive polymers are often called “the fourth generation plastic”, which are characterized by performing an active role like organic semiconductors instead of a passive role like insulators or the like. Such conductive polymers can be used depending on their conductivity for antistatic materials with a conductivity of from 10−13 S/cm to 10−7 S/cm, static discharge materials with a conductivity of from 10−6 S/cm to 10−2 S/cm, and EMI shielding materials, battery electrodes, semiconductors, or solar cells with a conductivity of 1 S/cm or more. If their conductivity is improved, the conductive polymers can be developed into further high-tech applications including transparent electrodes and the like.
Main conductive polymers known so far include a polyaniline, a polypyrrole, a polythiophene, a poly(p-phenylene vinylene), a poly(p-phenylene), and a polyphenylene sulfide (PPS).
Polythiophenes have been commercialized and widely used as poly (3,4-ethylenedioxythiophene-)- (PEDOT) (EP Patent No. 339 340) having a substituent in a thiophene ring. A chemical structure of the polythiophene is as shown below:

Polyanilines are an organic polymer having an alternating ring heteroatom backbone structure in which various substituents can be introduced to a benzene ring or a nitrogen atom, and can be classified depending on their oxidation state into a partially oxidized Emeraldine Base (EB) (y=0.5), a fully reduced Leucoemeraldine Base (LE) (y=1.0), and a fully oxidized Pernigraniline Base (PN) (y=0.0) as shown in the following structures.

These conductive polymers can be doped and dedoped through an acid-base reaction in addition to an electric method. In particular, conductivity of a polyaniline can be adjusted by using such an acid-base reaction and thus has been widely used. However, a kind of an acid may highly affect not only conductivity but also heat-resistant and environment-resistant stability. The polyaniline has two nitrogen atom groups in the backbone, and pKa values of the groups (—NH2+—) and (—NH+═) are 2.5 and 5.5, respectively. Therefore, a strong acid having a pKa <2.5 may donate protons to these two groups and can dope the polyaniline. An imine nitrogen atom of the latter can be fully or partially protonated in a protonic acid aqueous solution. In this case, it becomes Emeraldine Salts (ES) of which a doping level can be adjusted, and conductivity of the ES in the forms of powder and film is sharply increased from 10−8 S/cm to 1˜1000 S/cm. Such a doping process is well understood through numerous studies, and it has been well known to be largely divided into primary doping and secondary doping using solvent, etc. According to a method for doping by changing a counter ion of a sulfonic acid dopant suggested by Cao et al. [Y, Smith P, Heeger A J. Synth Met 1993; 55-57: pp 3514] as the most noteworthy method, solubility of a conductive polymer composite in an organic solvent is increased and processability is increased. However, with a low molecular weight (intrinsic viscosity of from 0.8 dl/g to 1.2 dl/g), a polyaniline can be dissolved in 1-methyl-2-pyrrolidone (NMP), and emeraldine salts doped with 10-camphorsulfonic acid (ES/CSA) can be dissolved in meta-cresol but can become gel at room temperature. Further, even if a conductive polymer blend is manufactured by using a relatively macromolecular organic acid such as dodecylbenzensulfonic acid (DBSA), acrylamidomethylsulfonic acid (2-acrylamido-2-methyl-1-propanesulfonic acid, AMPSA), camphorsulfonic acid (CSA), there is still a problem with environment-resistance or heat-resistance. In particular, as for a polyaniline product in the form of a film, a decrease in conductivity caused by loss of a dopant in the air is an important issue.
Besides, a polymerization method in which surfactants (micelles) or stabilizers or dopants DEHEPSA (di-2-ethylhexylester of 1,2 benzene dicarboxylic-4-sulfonic acid; Kulszewicz-Bajer et al., Synthetic Metals, 101, 1999, pp. 713-714) as plasticizers are added, a dispersion polymerization method in which polyvinyl alcohol is used as a steric stabilizer [J. Stejskal et al, Polymer, 37, 1996, pp 367], and a method in which such elements are added to already-polymerized polyaniline to improve processability and stability have been disclosed. However, according to these methods, electrical conductivity of a conductive polymer composite is sharply decreased.
M. Jayakannan et al. (US Patent Application No. US2009/0314995) and Paul et al. (U.S. Pat. No. 6,552,107) describe a method of preparing a cardanol-based derivative to be used as a dopant. According to each of them, an azo sulfonic acid derivative and a 3-pentadecyl phenol derivative are main structures of dopants, and a hydroxyl group and an alkyl side chain are introduced thereto, and, thus, solubility is increased along with regeneration potential using a cashew nut shell as a natural substance. However, an azo group can be thermally denaturalized and an alkyl group as a side chain is not well defined and a double bond may exist. Thus, chemical and physical characteristics can be changed. Further, there is also disclosed a regenerable lignosulfonic acid-based dopant (U.S. Pat. Nos. 5,968,417, 6,299,800, 6,764,617, etc.). However, most of the above-described dopants have electrical conductivity, as a main characteristic, as low as 10−3 S/cm, and, thus, there are limits in effectively using it.
Further, lots of research on polymer dopants has been carried out. Polymers used in such research may be polymers containing, for example, polyacrylic acid, polysulfonic acid, cellulose sulfonic acid, polyamic acid, polymer phosphoric acid, —COCl, or —SO2Cl.
Makowski H. S. [U.S. Pat. No. 3,870,841 (1975)] disclosed a method for preparing polystyrene sulfonic acid (PSS) from a polymer sulfonic acid derivative. PSS has been used in various uses such as insoluble polymer compounds, ion exchange resins, reverse osmosis, ultra filtering, plasticizers, and the like. As for a conductive polymer composite, PSS has been mainly used to prepare PEDOT. However, the PSS can act as a water-soluble polymer dispersion, and, thus, there are limits in applications. Further, since the PSS has a structure in which a sulfonic acid group is directly bonded to a benzene ring, if it is used as a dopant in a solid skeleton having a high molecular weight such as a polyaniline, mechanical properties of the conductive polymer composite can be weakened due to brittleness of polystyrene. Furthermore, a benzene ring of the polystyrene is stacked with a benzene ring of aniline, and in this case, the benzene ring of the polystyrene is changed depending on distribution of sulfonic acid groups introduced along polymer chains and non-uniformity between the chains may affect properties. Moreover, if a content of the sulfonic acid groups is increased, two sulfonic acid groups between molecules or in a molecule are cross-linked at a high temperature, so that sulfonation occurs between the benzene rings. Thus, the PSS is not appropriate as a dopant.